Problems arise in the making of electrical assemblies including circuits that create logic signals whose value is defined by the sign of the potential difference between two output lines of these circuits.
These signals are generally applied to the inputs of other circuits in the assembly, and these circuits can themselves furnish differential signals intended for other circuits located downstream.
These output signals are created by an output stage able to furnish sufficient power to supply the input stages of the downstream circuits. In the case of differential binary signals, differential amplifiers are often used as the output stage.
For performance reasons, particularly in MOS or CMOS technology, "precharging" circuits are commonly used. During a first "precharging" phase the differential lines are charged with a given voltage then, during a second "evaluation" phase the lines are charged to voltages defining a logic state which is a function of the input signals applied to the circuit. Thus, in the case of differential signals, when one of the lines is evaluated at a first voltage, the supply voltage for example, the other line is charged to a second voltage, the ground voltage for example. In this case, the attempt is made to precharge the lines to a voltage close to half the supply voltage in order to reduce the line switching time. Precharging is then generally accomplished by means of a precharging circuit which, under the control of a precharging signal, places the lines in communication with an auxiliary supply supplying a voltage which is half the supply voltage. Usually, the precharging circuit has an equalizing circuit to short-circuit the two lines to ensure that the precharging voltages of these lines are equal.
However, this classic solution has the drawback of requiring an auxiliary power supply whose voltage is difficult to adjust precisely, particularly in the case of integrated circuits. This is because integrated circuits have wide manufacturing differences and it is not easy to modify the characteristics of some of their subassemblies after manufacture.